1. Field of the Invention
The present invention relates to a sheet handling apparatus for storing sheets with plural sheet storage units.
2. Related Background Art
In the conventional image forming system consisting of an image forming apparatus (main body) connected with a recycling document feeder (RDF) and a sheet post-handling apparatus generally called a sorter, the operation of sorting a predetermined number of copy sheets prepared from a certain number of originals is achieved by placing the originals on the tray of RDF, selecting a sorting mode by a key of the main body, entering the required number of copies and depressing the copy start button. In response the lowermost (or uppermost) original is separated from the stacked originals, supplied onto the platen glass of the main body and is scanned by a number of times corresponding to the entered number of copies, whereby the copies of the required number are prepared. The prepared copy sheets are distributed, in succession, in the storage bins of a corresponding number in the sorter, starting from the first bin thereof. The above-explained operations are repeated for the number of originals, thereby achieving the sorting operations of the copies of the entered number.
Such conventional configuration may, however, run into a difficulty if the maximum number of originals that can be placed on the RDF does not coincide with that of sheets acceptable in each bin of the sorter, for example, in case a system with an RDF capable of accepting more than 50 originals is connected with a sorter of which each bin can store 50 sheets at maximum. No problems will arise as long as the number of the set originals does not exceed 50 since it is within the storage capacity per bin of the sorter. However, if the number of the originals exceeds 50, control means of the sorter sends a signal to the main body to inhibit the copying operation thereafter, when sheet counting means, such as a sheet sensor, in the sorter detects that the counted number of sheets has reached the maximum number, and the copying operations for the remaining originals can only be re-started by actuating the copy start button again after the copy sheets are removed from the bins of the sorter. Thus, the operator may find the copying operation being interrupted when he leaves the copying apparatus unattended and returns thereto after a sufficient time required for the copying, and is required to remove the copy sheets from the bins of the sorter and to temporarily store these copy sheets in a divided state, in order to re-start the copying operation. Moreover, after the remaining copying operation, there is required a cumbersome operation of matching the bundles of the temporarily stored copy sheets with those of the remaining copies, in order to obtain complete sets of the copies.
In order to prevent the above-mentioned situation, it is required to divide in advance the copying operation into two or more operations in such a manner that the number of the originals stacked on the RDF does not exceed the storage capacity per bin of the sorter, or to design the sorter in such a manner that the storage capacity per bin is at least equal to the maximum number of originals acceptable by the RDF.
In the former case, the number of the originals has to be known in advance. The originals can be counted by the operator or by the RDF, but this counting operation is required for each copying operation. For example, there can be conceived a method of requesting the operator to count and enter the number of the originals even for the originals of which number is evidently less than the maximum storage capacity per bin of the sorter, or a method of counting the originals by idle circulation of the RDF in advance, or a method, in case the number of the originals is identified as in excess of the maximum storage capacity per bin after such counting, of dividing the originals for effecting two or more copying operations or of storing the copy sheets in every other bin, in order to store the copy sheets in the adjacent empty bin when a bin becomes full, thereby accommodating a set of copy sheets in two adjacent bins. However, for executing such methods, there is always required a cumbersome operation of counting the originals in advance. Such operations involve a significant loss in time, so that an improved throughput cannot be expected in an automated system.
In the latter case, if the bins are designed with a limited space, an increase in the storage capacity per bin results in a reduced number of bins. As an example, if 20 bins capable of storing 50 sheets each can be provided in the given space, there can be provided only 10 bins capable of storing 100 sheets each. Consequently, the settable number of copies becomes limited for the frequently encountered number of originals, which is usually less than 50. This means that a new drawback is generated by resolving the above-mentioned drawback that the maximum settable number of originals exceeds the maximum storage capacity per bin of the sorter.
In this manner, the conventional configuration has been associated with various drawbacks such as the complication of the copying operation resulting from the fact that the number of originals is not known in advance, or the counting means for the number of originals being unacceptable due to the loss in time in the copying operation.
The original recycling systems can be generally classified into following three types.
A first type is the switch-back original feeding method, in which an original sheet is supplied from an original tray to an image reading position on a platen glass from an end thereof, then read by the movement of an image reading unit of the image forming apparatus after the sheet is placed in a predetermined position, and, after the image reading, the sheet is discharged through the same end of the platen glass to the tray.
In such method, the time required for sheet exchange (hereinafter called sheet exchange time), after the reading of the image of the sheet by the image reading unit, from the sheet discharge from the platen glass to the placing of a next sheet on the platen becomes long because there is involved a transport distance of about two sheets for the sheet discharge from the platen and the supply of the next sheet.
Consequently, in a high-speed image forming apparatus, since the between-sheet time (distance between the rear end of a sheet and the front end of a next sheet, divided by the process speed) becomes shorter with the increase in the speed, the productivity of the image forming apparatus in a 1-to-1 image formation (forming an image from a sheet) cannot be made 100% unless a relation [sheet exchange time].ltoreq.[between-sheet time] stands.
For this reason, the above-mentioned switchback method is generally considered unable to achieve a productivity of 100% in a high-speed image forming apparatus because of the long sheet exchange time.
However, such switchback method, being capable of feeding the originals from a direction close to the home position of the image reading optical system of the image forming apparatus, has the advantage that the distance from the original stacking tray to the feed position on the platen is relatively short, so that the time required from the start of separation of the sheet to the placement thereof on the platen glass and the time to the start of first copying can be shortened.
A second type is the feeding method with a closed-loop original feeding device. The sheet is fed to the image reading position on the platen glass from an end thereof, and, after image reading, it is discharged, depending on the sheet size, either from the same end of the platen glass to the sheet tray or from the opposite end of the platen glass to the sheet tray through a closed-loop sheet path. Thus, a large-sized (for example, A3) sheet is transported by the switchback method as explained above, but a small-sized sheet (for example, A4 or smaller) is transported through said closed-loop path.
Such closed-loop method can achieve high-speed sheet exchange in comparison with the switchback sheet feeding, because the sheet exchange only involves a transport distance corresponding to a sheet and a between-sheet distance. However, an increase in the transport speed may cause difficulty in controlling the precise stopping position, thereby resulting in more damage to the sheets due to sheet jamming, an increased size of the motor leading to a larger size of the apparatus, a higher cost thereof and an increased level of noise.
A third type is the document feeder capable of switchback feeding and non-stop image reading by a closed loop, in which so-called non-stop image reading is effected by fixing the image reading unit of the image forming apparatus and continuously transporting the original sheet for achieving a high-speed process.
In such non-stop image reading, the image reading has to be executed while the original sheet is transported from an end of the platen glass toward the other end, so there is provided another sheet feeding slot for feeding the originals from the opposite side to the switchback path, thereby enabling to feed the sheet to the image reading position from either end of the platen glass.
In such a document feeding device, the switchback feeding and the closed-loop non-stop image reading are both used for feeding the sheet from an optimum direction, according to the operation mode. Since control means is provided for switching the fixed sheet reading mode and the non-stop sheet reading mode, the sheet exchange time can be made equal to or less than the between-sheet time of the image forming sheets in the image forming apparatus, by employing the non-stop image reading mode for a 1-to-1 copying operation with a half original size or smaller. Consequently, such a document feeder, even on a high-speed image forming apparatus, can achieve a productivity of 100% without sacrificing the copying speed in a 1-to-1 copying operation for a half-sized sheet, namely without requiring a high-speed handling of the original sheets.
Also, in a 1-to-1 copying mode, a process ability same as mentioned above can be achieved by feeding the originals in continuation through the closed-loop path, which feeds the originals from the opposite direction.
Also, in the field of sorters, there have been proposed various sorters responding to the diversifying requirements of the users.
For example, certain users require to prepare a large number of copies from a relatively limited number of originals, while other users require to prepare a limited number of copies from a large number of originals. For the former users, there are required a large number of storage bins though the sheet storage capacity per bin is limited, while, for the latter users, many bins are unnecessary if the storage capacity per bin is high.
In order to accommodate these contradicting requirements, there are required a large number of bins each having a large sheet storage capacity, and such configuration eventually leads to the use of two sorters or a large-dimensioned sorter, giving rise to a high cost.
For example, in case of preparing a set of copies from a small number of originals (several originals or less), even if the image reading is conducted in the non-stop image reading mode in which the sheet exchange time is shortest, the advantage of such non-stop image reading (flow reading) does not become obvious, because the time to the start of the first copying operation is longer in the non-stop image reading path than in the switchback path or in the closed-loop path.
For this reason, if a document feeder capable of switchback feeding and non-stop image reading with a closed loop path is mounted on a high-speed image forming apparatus, in a 1-to-1 copying operation of preparing a copy only from a limited number of originals, the total copying time, from the actuation of the copy start button to the completion of the copying operation, is strongly influenced by the time required to the start of the first copying operation, and the total copying time may be shorter in the switchback mode which has a longer original exchange time than in the non-stop image reading mode.
Since such inconvenience cannot be recognized by the document feeder until all the originals placed thereon are recycled, such document feeder may result in a loss of productivity in case of preparing a copy only (or sometimes two copies) from a limited number of originals.